The concept of "Extra Quality" in this context refers to the specific compilation flags and decoding precision settings enabled within the custom codec. Unlike native Android audio frameworks (AudioFlinger) which may truncate audio to 16-bit for power saving, custom FFmpeg builds can be compiled to support:

If rooted, install ViPER4Android FX . Route MX Player’s output through ViPER. The combination of EAC3 custom codec + ViPER’s convolver will give you studio-quality sound.

EAC3 (Enhanced AC-3) is designed for high-definition audio streaming. It uses a superior compression algorithm to deliver even over lower bandwidth connections. Without the custom codec, MX Player often plays the video in silence or throws an "Audio format EAC3 not supported" error. How to Unlock This "Feature"